1. Technical Field
The invention relates to an optical element molding method and an optical element.
2. Description of the Related Art
In recent years, as optical devices have become smaller and more lightweight and have had more functions, various optical lenses used for optical systems have been developed. Particularly, in products employing a pick-up lens for an optical disk such as a DVD (Digital Versatile Disk), an increase in NA of the optical lens has been required. Also, recently, in a Blu-ray disk (a mass storage optical disk), a high NA lens has been used together with a blue-violet laser having a short wavelength in order to achieve high-density data recording. Thus, as might be expected, an increase in NA of the optical lens will be further required in the future. In addition, such a pick-up lens, for example, is required to have 0.45 to 0.5 NA for a CD (Compact Disk), 0.6 to 0.65 NA for the DVD, and 0.85 or more NA for the Blu-ray disk.
As a method of molding an optical lens (hereinafter, it is referred to as “optical element”), the most widely used is a press molding method of molding an optical element from a molding material by using a pair of molds having transfer surfaces including an optical transfer surface and a barrel die to which the molds are inserted. In the press molding method, the molding material is placed on a first mold, the molding material is pressed in a heat-softened state by the first and second molds, the transfer surfaces are transferred, and then the molding material is cooled. Thus, a desired optical element is molded.
In this case, an optical function surface of the optical element is defined as a range including an effective diameter (a range through which effective rays pass through the optical system) of the optical element and outside thereof. If a molding process is performed only for the range of the effective diameter, it is difficult to process the molding material in accordance with a designed shape in order to achieve a function of the optical element. Thus, the optical function surface is defined as a range in which the optical element is molded in accordance with the predetermined design shape for achieving a function of the optical element together with the range of the effective diameter thereof.
In addition, a radius of curvature in the case where the optical element is formed in an aspheric surface is defined as a radius of curvature in the vicinity of an optical axis of the optical element. Likewise, in the transfer surface of the mold and the surface of the molding material used for molding the optical element having an aspheric surface, the radius of curvature is defined as a radius of curvature in the vicinity of an optical axis of the optical element.
Generally, in order to response the requirement of high NA with a single lens, it is necessary to increase an effective diameter of an optical element and utilize even a highly inclined part on a lens surface as an effective diameter. Thus, a thickness of an outer peripheral part in the optical element decreases. Hence, in order to secure a thickness of the outer peripheral part necessary to process the optical element, a thickness in the vicinity of the optical axis of the optical element should increase. Thus, volume of the molding material increases. As a result, when the radius of curvature of the molding material becomes larger than the radius of curvature of the transfer surface, a closed space is formed between the molding material and the transfer surface of the mold in a state where the molding material is placed thereon. Thus, an appearance defect such as a recess is easily generated in the optical surface of the molded optical element.
To solve the problems mentioned above, JP 2004-335080 A (corresponding to US 2005/0053788 A and US 2007/0253073 A) describes a manufacturing method of molding a lens having a convex aspheric surface and having 0.8 or more NA with a high accuracy in lens surface. In this manufacturing method, by using a pair of the molds having transfer surfaces opposed to each other, the spherical molding material having a radius r is heat-softened and pressed, and a transfer surface is transferred. Thus, a desired lens is molded. When a lens having a convex aspheric surface of a radius of curvature R is molded, a condition that r/R≦1.3 is satisfied and a molding condition for properly suppressing pressing speed is proposed. Also, by satisfying the molding condition, it is effectively prevented to mold a lens in a state where air is trapped between the molding material and the transfer surface of the mold. Thus, it is possible to mold a lens having high accuracy in lens surface.
However, in a verification experiment performed by inventors, it was found that, even if the molding condition described in JP 2004-335080 A is satisfied, sometimes, a lens may have an appearance defect such as a recess due to molding of the lens in a state where gas is trapped between the molding material and the transfer surface of the mold.